The present invention relates generally to superconducting wire and tape, and more particularly to a process for making a neodymium gallate (NGO) surface layer on the inside surface of a protective outer sheath of nickel surrounding a core of yttrium-based copper oxide (YBCO) superconductor material. The NGO surface layer acts as a diffusion barrier to prevent diffusion of nickel into the YBCO during high temperature melt-processing.
High temperature superconductors are typically copper oxide ceramics which have little mechanical strength. They must be sheathed by compatible metals such as silver or silver alloys in order to be mechanically sound and to provide thermal stability and quench protection. Some bismuth-based copper oxide ceramic superconductors work well with silver or silver alloy sheaths because grain growth and necessary texturing can be conducted by mechanical means with subsequent sintering at relatively low temperatures around 85xc2x0 C. to create a valuable superconducting wire or tape for winding magnets. Work along these lines has been successfully pursued by the inventors and others worldwide. Cylindrically sheathed conductors, or superconducting wires, have been produced with critical current densities in magnetic fields of 6xc3x97104 A/cm2 at 20xc2x0 Kelvin. Unfortunately, these bismuth-based copper oxide superconductors do not perform well at high temperatures ( greater than 30xc2x0 K) and high magnetic fields because they lack good magnetic flux pinning.
On the other hand, YBCO superconductors can perform well at high temperatures. However, YBCO superconductors do not develop the prerequisite grain growth and texturing necessary for good current density unless they undergo a melt process in the vicinity of 1050xc2x0 C. Consequently, a sheath encasing YBCO must also withstand this high melt temperature without interacting with the YBCO. Silver and a few silver alloys are the only metals which can be directly applied as a sheath to YBCO without poisoning superconductor performance. Unfortunately, these metals tend to melt at temperatures near 1100xc2x0 C. There is a need, therefore, for metal sheaths or cladding which can survive the high temperatures of YBCO melt-processing. Nickel and nickel-based alloys have been tested by the inventors and found to be good sheath candidates. Some nickel-sheathed YBCO conductors fabricated and tested by the inventors have carried 1400 amperes, a world record. The problem with a nickel sheath is that nickel diffuses into the YBCO during the very hot melt process and replaces copper in the YBCO. When the nickel for copper exchange occurs, the critical temperature of the YBCO is significantly depressed so that the critical current density begins to disappear above 40xc2x0 K, even though excellent magnetic flux pinning has been introduced into the YBCO by a variety of different methods. Despite its promise, this level of superconductor performance has thus far been no better than other approaches above 40xc2x0 K.
Thus it is seen that there is a need for a method for preventing nickel diffusion from a nickel sheath into YBCO during melt-processing and at other times.
It is, therefore, a principal object of the present invention to provide a nickel diffusion barrier for a nickel sheath to enhance YBCO superconductor performance.
It is another object of the present invention to provide a nickel diffusion barrier for a nickel sheath for any ceramic superconductor material.
It is a feature of the present invention that it can be used to provide a nickel diffusion barrier for any nickel substrate, such as a tube, a flat tape, a wire, a plate or other shapes.
It is another feature of the present invention that it permits forming of superconducting wires or tapes with very high critical current densities.
It is an advantage of the present invention that it makes possible the manufacture of YBCO superconductor wire.
It is another advantage of the present invention that simplifies the process of making superconductor wire by allowing the nickel sheath to act as a crucible for the demanding and necessary melt-processing of YBCO.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.
The present invention provides a neodymium gallate surface layer on a nickel sheath as a diffusion barrier to prevent diffusion of nickel into YBCO during high temperature melt-processing of the YBCO. The unique discovery of the present invention is that a neodymium gallate (NGO) surface layer on a nickel sheath will prevent nickel diffusion during melt-processing of the YBCO. The key step is presintering a surface layer of NGO powder applied over the nickel sheath at temperatures sufficient for nickel from the nickel sheath to diffuse into the NGO.
Accordingly, the present invention is directed to a ceramic superconductor comprising a nickel substrate, a yttrium-based copper oxide superconductor and, between the nickel substrate and the yttrium-based copper oxide superconductor, a diffusion barrier made of neodymium gallate, wherein the neodymium gallate has been presintered at temperatures above about 1000xc2x0 C. The presintering may be performed at temperatures in the range of about 1000xc2x0 C. to about 1300xc2x0 C. The nickel substrate may be a tube, a flat tape, a wire or a plate.
The present invention is also directed to a method for making a substrate for a ceramic superconductor, comprising the steps of providing a nickel substrate, applying a layer of neodymium gallate over the nickel substrate as a diffusion barrier and sintering the layer of neodymium gallate at temperatures above about 1000xc2x0 C. The layer of neodymium gallate may be sintered at temperatures in the range of about 1000xc2x0 C. to about 1300xc2x0 C. The nickel substrate may be a tube, a flat tape, a wire, or a plate.
The present invention is additionally directed to a method for making a ceramic superconductor, comprising the steps of providing a nickel substrate, applying a layer of neodymium gallate over the nickel substrate, sintering the layer of neodymium gallate at temperatures above about 1000xc2x0 C., and applying a layer of yttrium-based copper oxide over the layer of sintered neodymium gallate. The layer of neodymium gallate may be sintered at temperatures in the range of about 1000xc2x0 C. to about 1300xc2x0 C. The nickel substrate may be a tube, a flat tape, a wire or a plate.